1. Field of the Invention
This invention relates to wireless communication systems, and more particularly to methods for performing cell selection handoffs in a wireless communication system.
2. Description of Related Art
Wireless communication systems facilitate two-way communication between a plurality of subscriber mobile radio stations or “mobile stations” and a fixed network infrastructure. Typically, the plurality of mobile stations communicate with the fixed network infrastructure via a plurality of fixed base stations. Exemplary systems include such mobile cellular telephone systems as Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, and Frequency Division Multiple Access (FDMA) systems. One objective of these digital wireless communication systems is to provide communication channels on demand between the mobile stations and the base stations in order to connect the mobile station users with the fixed network infrastructure (typically a wired-line system).
Exemplary CDMA Communication System
Mobile stations typically communicate with base stations using a duplexing scheme that allows for the exchange of information in two directions of communication. In most existing communication systems, transmissions from a base station to a mobile station are referred to as “forward link” transmissions. Transmissions from a mobile station to a base station are referred to as “reverse link” transmissions. Code Division Multiple Access, or “CDMA”, systems are well known in the art. For example, one such system is described in U.S. Pat. No. 4,901,307, issued on Feb. 13, 1990 to Gilhousen et al., which is hereby incorporated by reference for its teachings of CDMA wireless communication systems.
Radio system parameters and call processing procedures for exemplary prior art CDMA systems are described in the TIA specification, entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” TIA/EIA/IS-95-A, published in May 1995 by the Telecommunications Industry Association, and is referred to hereafter as “IS-95A”. An update and revision to IS-95A and J-STD-008 (PCS specification analogous to IS-95A) is the TIA/EIA/IS-95-B standard, which was published in March 1999 by the Telecommunications Industry Association (TIA), and is referred to hereafter as “IS-95B”. The IS-95A and IS-95B specifications jointly specify second generation or “2G” CDMA systems. More recently, a third generation, or “3G” CDMA system, has been described in the TIA specification, and is entitled “cdma2000 Series”, TIA/EIA/IS-2000-A. The TIA/EIA/IS-2000-A specification was published in March 2000 by the TIA, and is referred to hereafter as “IS-2000”. The IS-95A, IS-95B and IS-2000 specifications are incorporated by reference herein for their teachings on CDMA communication systems.
FIG. 1 is a simplified depiction of a coverage area in a CDMA communication system having a service area comprising seven base stations, wherein the base stations are controlled by a single Mobile Telecommunication Switching Office (MTSO) 20. As shown in FIG. 1, a typical CDMA communication system comprises at least one mobile station and a plurality of fixed base stations geographically distributed over the system service area and controlled by the MTSO 20. A service area is defined as the geographical area within which a mobile station can remain and yet still maintain communication (i.e., maintain a valid radio link) with the CDMA communication system. Each base station provides communication services to a fixed area within the service area. The service area is known as the base station “coverage area”. When a mobile station is within a base station's coverage area, the base station is able to provide communication services to and from the mobile station. Base stations that provide service to a mobile station are also known as “serving” base stations. The MTSO 20 coordinates all of the switching functions between the base stations, mobile stations, and other communications systems (e.g., a Public Service Telephone Network or satellite communication system).
Communication between base stations and mobile stations is established using a negotiation process that is initiated upon call origination. The serving base station begins the negotiation process by assigning a selected one of its available forward traffic channels to the mobile station and thus establishes a forward link with the mobile station. The mobile station then establishes a reverse link with the serving base station. Once communication is established between a serving base station and a mobile station, “pilot” channels emitted by each base station are used by the mobile station to identify the base station coverage area in which the mobile station belongs. The mobile station also uses the pilot channels to determine the quality of the radio link between the mobile station and the base station. Specifically, each base station transmits an unmodulated pilot channel on a predetermined frequency that aids the mobile stations in detecting signals and measuring signal strengths of nearby base stations.
CDMA Handoffs
CDMA handoffs occur when a mobile station moves from the coverage area of its active base station to the coverage area of a new base station. In typical CDMA systems, a mobile station maintains a list of available base stations for providing communication services to the mobile station. Normally, the mobile station communicates with base stations that have strong signals. The mobile station receives the pilot signals and determines which pilot signals are the strongest. A “searcher” unit in the mobile station commonly performs these signal detection and strength measurement functions.
The results of the searcher function are reported to the current (i.e., the active) base stations. The base station then instructs the mobile station to update a list of available base stations that are maintained by the mobile station. The list is sub-divided into three operative sets—an “active set”, a “candidate set”, and a “neighbor set”. The active set contains a list of the base stations with which the mobile station is currently communicating. The candidate set contains a list of base stations that may move into the active set. The neighbor set contains a list of base stations that are being monitored, albeit on a less frequent basis.
As the mobile station moves and its currently active base station signal weakens, the mobile station must access a new base station. Based upon the results of the searcher function, and the instructions received from the base station, the mobile station updates its sets, and communicates with a different base station(s). In order for communication transmissions to appear seamless to the user of the mobile station (a highly desirable goal), the communication link must be “handed off” to the next base station. A handoff occurs when a mobile station moves across a “boundary line” from a first serving base station coverage area to a second base station coverage area. The communication system “hands off”, or “hands over”, or transfers service from the first serving base station to the second base station, also known as the “target” base station. A handoff also occurs when a single base station utilizes multiple frequency channels and switches communication between frequency channels. Each pilot channel is identified by a pseudo-random noise (PN) sequence offset and a frequency assignment. Thus, each pilot channel is uniquely identified with a base station that transmits the pilot channel. Thus, the pilot channels aid the mobile stations in performing handoffs between base stations.
Referring again to FIG. 1, each base station services a separate coverage area, roughly approximated and represented by a hexagon, and communicates using a specific frequency, a frequency one (F1) or a frequency two (F2). Examples of wideband channels used by CDMA systems include the well-known Cellular (800 MHz) and PCS (1900 MHz) bands. Other wideband channels can be used without departing from the scope or spirit of the present invention. In the exemplary CDMA system of FIG. 1, a first base station 12, located approximately in the middle of a Service Coverage Area One, communicates on a first frequency F1. A mobile station 10 is located in the Coverage Area One and therefore is serviced by the first base station 12. When the mobile station 10 moves from the Coverage Area One to a Coverage Area Two, it performs a handoff procedure from the first base station 12 (the serving base station) to a second base station 14 (the target base station). Thus, the moved mobile station 10′ is thereafter serviced by the second base station 14.
There are two basic types of handoffs in CDMA systems: “hard handoffs” (HHO) and “soft handoffs” (SHO). As is well known, a “soft handoff”, or “Make-Before-Break” handoff, is a handoff procedure in which the mobile station initiates communication with a target base station without interrupting communication with a serving base station. Because mobile stations typically include only one transmitter, soft handoffs typically are only used between base stations with CDMA Channels having identical frequency assignments. Referring again to FIG. 1, a soft handoff procedure is performed when the mobile station 10 travels from a first Coverage Area One to a third Coverage Area Three because the base station 12 and a third base station 16 have identical frequency assignments, F1 as shown in FIG. 1.
A “hard handoff” is defined as a handoff in which a mobile station initiates communication with a target base station after a momentary interruption in communication with a serving base station. Hard handoffs are also commonly referred to as “Break-Before-Make” handoffs. A hard handoff is typically used when the serving base stations and the target base stations use differing CDMA channel frequency assignments. As shown in FIG. 1, the first base station 12 uses a first frequency F1 and the second base station 14 uses a second frequency F2. A hard handoff is performed when the mobile station 10 travels from the Coverage Area One to the Coverage Area Two (101) because the first base station 12 and the second base station 14 transmit and receive using different frequencies, specifically, using F1 and F2, as shown in FIG. 1.
A hard handoff can also occur when a single base station uses multiple frequency channels and switches communication between its frequency channels. For example, a single base station hard handoff can occur between sectors associated with a single base station. The present invention addresses problems associated with the multiple base station scenario, and thus, the single base station scenario is not described in great detail herein. However, those skilled in the art shall recognize that the present invention can be utilized equally as well in a single base station scenario.
During a hard handoff operation, the radio link is momentarily interrupted because, as described above, a typical mobile station includes only one transmitter, and therefore, a typical mobile station can demodulate only one frequency at a time. Thus, switching from the CDMA channels of the serving base station (operating, for example, at a first frequency F1), to the CDMA channels of the target base station (operating, for example, at a second frequency F2), produces a momentary interruption in the continuity of the radio link between the mobile station and the base stations.
Disadvantageously, interference to the receivers (e.g., mobile stations) in wireless communication systems (e.g., CDMA communication systems) is increased during a SHO because multiple base stations maintain transmissions during a SHO. Prior art methods attempting to overcome the increased interference problem that is associated with multiple base station transmissions during SHO have several disadvantages. First, these methods are not capable of selectively gating (i.e., shutting off) base stations during selected time intervals. Specifically, these methods cannot gate off weak, unnecessary (i.e., base stations that are not necessary to achieve a desired QoS) and excess base stations. Excess base stations are defined as base stations that exceed the physical limitations of a receiver (i.e., the number of demodulation fingers in a receiver). For example, a receiver having four demodulation fingers is physically limited to demodulating signals from a maximum of four selected base stations, and thus base stations other than the four selected base stations are excess base stations. Second, communication systems utilizing prior art methods cannot mitigate short-term fading problems.
Therefore, a need exists for a method and apparatus for reducing interference during SHO to receivers in wireless communication systems. Such a method should increase the voice and/or data capacity in wireless communication systems. In addition, the inventive method should be able to selectively gate off base stations (e.g., weak, unnecessary and excess base stations) during selected time intervals. Further, the method and apparatus should be able to reduce or minimize short-term fading problems. The present invention provides such a method and apparatus for performing cell selection handoffs in wireless communication systems.